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An Educated Look At Weeds
by Dan Clost
by Dan Clost

email: dan.clost@sympatico.ca

First serious garden earned 25 cents from the Kemptville Horticultural Society when I was 12. Have been poor in horticulture ever since but rich in spirit.

Went to work writing the Good Earth column (over 500 articles published in newspaper, magazine, website and journal.) and learned that what was printed wasn't what I wanted to say and certainly not what Gentle Reader understood me to say. Subsequently have developed a certain clarity and economy of words.

Day job- nursery and production manager for a large nursery/garden centre
Side job- Garden restoration and renovations, design consultations, remedial pruning.
Night job- garden writer and communicator (overnight success in another 20 years)

Dan gardens in Canadian Zone 5b


April 13, 2003

Along with swelling buds, and greening grass we are also noticing weeds so now is a good time to learn a little bit about these so-called enemies. To help us out, I met with Wendy Asbil. She asked to be identified as a lecturer/researcher in pest management, mostly weeds and forages, cereals and alternative crops. A rather broad spectrum, wouldn't you agree? So I can't tell you about her academic qualifications, her research papers, her involvement in biorational controls and a hockeysock full of other really impressive stuff. I can tell you that, as a research scientist and lecturer at the University of Guelph, there are very few persons her equal.

Wendy has the following description of a weed: there are several definitions of a weed. It is a plant that has a negative impact on crop production (like our front lawn) and animals (like people). It competes with desired plants, reduces quality/value. It can harbour diseases and harmful insects. Weeds are always part of production, gardening, etc. They are plants that we strive to control but cannot expect to eliminate.

(Commentary: Sort of a comprehensive answer isn't it? There are plants out there that qualify as both a desired species and as a weed, depending upon who is doing the naming. Evening Primrose is one of those. In a properly tended residential setting, it is easily controlled and provides three seasons of colour.)

She pointed out that the presence of a particular weed gives us quite a bit of information. They can be indicators of soil conditions such as pH [plantain & sorrel like high acid content], high organic matter [groundsels], poor drainage [horsetail] and compaction [knotweed].

(Commentary: And then there are some that will grow just about everywhere and under just about every condition. Dandelions are the one that pops up in my mind.)

By understanding these indicators we know that we can change the weed spectrum by altering soil conditions. We can aerate, drain, feed or add lime. Anything that will keep the weed off-kilter so that they are less likely to become established.

Generally weeds shouldn't cause too much concern unless they are new weeds that you haven't seen before. These can be the result of unclean seed or plant materials, manure, topsoil, compost or they can be the result of propagules brought in by wind, animals, your own clothes and vehicles. (Commentary: Propagules are any bit of a plant that can produce a new plant: bulbs, cuttings, rhizomes even a little piece of a succulent stem, like our friend purslane. We know all about the wind and dandelions floating in on their little parachutes (pappus) and we have picked enough burdock and beggar-ticks seeds off of our trousers and shoelaces to have a very good understanding of a weed's ability to travel and see a bit of the world before settling down to raise its own generation of progeny. Have you thought about those cute little birds that are attracted to your feeder? Some seeds sort of scoot through the digestive tract after being nicely scratched (scarified) in the crop. A small price to pay for their cheerful presence but one we can be aware of practices that will help us stay below our chemical-action threshold.)

Weeds in a flowerbed or vegetable garden are relatively easy to control. Hand pulling is the cheapest and easiest method unless there is an overwhelming infestation. It is important to remove all weed debris from the garden. Some of the little devils, like the succulent purslane, can quite easily reproduce itself from the smallest piece of stem if left lying about.

The second, and the best, is to employ properly applied mulch. Weed suppression is only one of the many benefits of its use. And yes, I do tend to go on about mulch, but I am convinced it is a gardener's best friend.

The absolute best method of weed control, to paraphrase Wendy, is to make grass the pioneer crop in your greensward. "Make grass the living mulch that suppresses weeds." Have an understanding of grass' cultural needs and then provide that environment.

Unfortunately, weeds are exceptionally tenacious and circumvent most of our efforts. They warrant a more detailed look in three distinct groups. The first is annuals. Crabgrass is one of the more well known although quite a few people think of it as a perennial scourge due to its repeated appearances. In our area (Eastern Ontario) we have two types, Large Crab Grass (Digitaria sanguinalis) and Smooth Crab Grass (Digitaria ischaemum). The differences are moot as control is the same: don't let their seed germinate. Deny them light by growing a dense healthy stand of grass. That's all it takes. In fact, others in this class, Common Mallow (Malva neglecta) Yellow Clover/Black Medic (Medicago lupilina) and Purslane (Portulaca oleracea), are susceptible to the same control. Hand pulling has the advantage of preventing seed set. The efficacy of this method depends on the size of the yard, the degree of infestation and how many munchkins are about that you can convince to do the job.

Perennials is the next category. Dandelion, or by its other sobriquets of Taraxacum officinale, dent-de-lion, and "first spring bouquet our children present to us" is the best known. We are aware of its long taproot and how easily it breaks when we try to spud them out. If this is your preferred method of control, Wendy suggests that you try doing it after rain or a watering. The combination of the tapered root and the "looser" damp soil increases the amount of root that will pull up.

However, if you only get the tops, don't be discouraged. This is the time of the year that the weed is at its most vulnerable. All perennials, even Creeping Charlie (Glechoma heraceae), are busily using up their nutrient reserves to grow these tops. Eventually, if you are as persistent as they, they will run out of food.

The last group is biennials, plants that require two growing seasons to complete their life cycle. The first year plant is usually a rosette or cluster of leaves very close to the ground and compact. Blueweed (Echium vulgare) and Common Mullein (Verbascum thapsus) are two familiar plants in this category. As Wendy points out, this is the easiest group to control because you have an entire growing season to take care of the vegetative parts. Mowing and hand pulling are the best methods.

If all else fails, and if you are philosophically inclined, chemical control is the heavy artillery. Proprietary products are available to the homeowner, usually as part of a fertilizer mix or a mixable product containing 2,4-D or glyphosate. Commercial applicators employ a wider range of chemicals and can treat large areas relatively economically. Whichever you use, please be aware that this stuff is exceptionally toxic and must be handled with consideration.

In closing, decide on your threshold acceptance level, identify the weeds and use an integrated approach that is the least intrusive to the environment.


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